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C 2 H 2 , HCN and SiO bands of the AGB stars in the LMC/SMC/Sgr

Mikako Matsuura (UMIST, UK) A.A. Zijlstra, J.Th. van Loon, I. Yamamura, A.J. Markwick, P.M. Woods, L.B.F.M. Waters. C 2 H 2 , HCN and SiO bands of the AGB stars in the LMC/SMC/Sgr. Introduction. Lower metallicity should affect the abundance of molecules in the AGB stars

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C 2 H 2 , HCN and SiO bands of the AGB stars in the LMC/SMC/Sgr

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  1. Mikako Matsuura (UMIST, UK) A.A. Zijlstra, J.Th. van Loon, I. Yamamura, A.J. Markwick, P.M. Woods, L.B.F.M. Waters C2H2, HCN and SiO bands of the AGB stars in the LMC/SMC/Sgr

  2. Introduction • Lower metallicity should affect the abundance of molecules in the AGB stars • Metallicities: [Z/H]= -0.3 (LMC), -0.7 (SMC), -0.55 (Sgr) • Molecular bands should be weaker in lower metallicity • L-band spectra • Oxygen-rich stars • SiO bands • Carbon-rich stars • HCN and C2H2 bands • SiO and C2H2 • parents molecules for the dust grains (silicate; PAH) • Abundance of these molecules should be related to amount of dust formed in the AGB stars

  3. Observations • VLT+ISAAC • Wavelength resolution and coverage • Oxygen-rich stars: R=2000-3300, 3.95-4.1 micron • Carbon-rich stars: R=360-600, entire L-band • Targets AGB stars (Long period variables) from Groenewegen & Blommaert (1998), van Loon et al. (1998), Whitelock et al. (1996) Oxygen-rich stars Carbon-rich stars LMC 4 7 SMC 0 1(+1post-AGB) Sgr 0 3 SMC S2 (post-AGB) PAH

  4. Spectra of carbon-rich stars HCN C2H2 HCN + C2H2 V Cyg (galactic c-star) IRAS 04496-6958 (LMC) SMC S30

  5. Solar Neighbour LMC Equivalent Widths SMC Sgr 3.1 micron HCN+C2H2 • HCN is most abundant in Galactic AGB stars • Large C2H2 abundance in LMC and SMC than in Solar neighbour 3.8 micron C2H2 3.5 micron HCN H-K

  6. Interpretation with chemical model • At the same Teff and C/O ratio, both HCN and C2H2 abundances are less at lower metallicity • Higher C/O ratio will explain more abundant C2H2 in LMC/SMC • The third-dredge up 2000 K [Z/H]=0.0 (Solar Neighbour) 2000K [Z/H]=-0.3 (LMC)

  7. Oxygen-rich stars Aringer et al (1999) for the galactic stars 28Si16O Galactic stars: semi-regular variables g Her (solar neighbour) ISO/SWS, R=2000 Mira variables 29Si16O IRAS 05042-6720 (LMC) R=3300 S/N is determined by the spectra of telluric standard LMC (Mira)

  8. Low Si abundance? Aringer et al. (1997) • Even lower SiO abundance than prediction in LMC • Additional effects? • Filling with SiO in the extended atmosphere? • Higher C/O ratio after the third dredge-up? Observed range LMC 12.0+log(Si/H)~7.0 = EW~30A (Dopita et al 1997) Solar neighbour 12.0+log(Si/H)~7.6 = EW~55A

  9. Summery • Carbon-rich stars • HCN is the most abundant in the solar neighbour • More abundant C2H2 in LMC/SMC • Oxygen-rich stars • Non detection of SiO bands • EW(SiO) is below the predicted level • Lower abundance? • Filling the absorption by SiO in extended atmosphere? • High C/O ratio in LMC/SMC after the third dredge-up • In lower metallicity, maybe less dust grains in oxygen-rich stars, but more dust grains (PAHs) in carbon-rich stars (maybe not in AGB stars but in post-AGB stars)

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